Joining objects in Blender 2.8 causes the objects to disapear
This looks like a bug. But you can use a helper object, to merge these objects.


Add a Curve Object. &#8679; ShiftA > Curve > Bezier
Go into edit mode &#8633; Tab and delete X the selected control points leaving you with an empty bezier. Exit edit mode &#8633; Tab.
Select all other objects with the border selection B. [Make sure, that the newly created (empty) bezier object stays the active object.]
Merge them with &#9096; CtrlJ.